The present invention relates generally to photography and pertains particularly to method and apparatus for retrieving information related to regional exposure thereby facilitating the enhancement of the image of photographic films, plates and the like.
Ordinary photography is based on the light sensitive properties of silver halides and more particularly silver bromide. In the unexposed state photographic films are made of an emulsion of silver bromide crystals and a suitable binder usually applied in a very thin layer on a suitable supporting surface such as a flexible plastic backing. The silver bromide particles are activated by exposure to light and are rendered reducable to elemental silver. The film is then developed by exposure of the film to chemical baths in a specific order. These baths are a reducing solution, a fixer, and a wash. The developing solution reduces the activated silver bromide particles to aggregates of metallic silver. These aggregates form the contrast necessary to image formation. The fixing bath washes or dissolves out of the emulsion the silver bromide which has not been reduced. The film is then dried to form what is referred to as a negative. A washing bath removes the last traces of the developer and the fixer. The silver left in the film as a result of this process forms the black and white contrast which subsequently forms the image.
Image retrieval and reconstruction commonly is accomplished by means of transillumination. This is carried out by placing the film between a light source and an observer so that the film acts as a filter. The effect is to produce regions of contrasting light and dark forming an image.
The problem with this form of image retrieval is that the relationship between the original exposure level and the resulting film opacity or ability to transmit light is very nonlinear. This is shown by for example the graph in FIG. 1 where the percent of absorption is plotted on the vertical axis against the log of exposure on the horizontal axis. Thus, from this graph it is seen that the exposure level to produce a satisfactory film is very critical. The linear range on the curve A of the graph extends between points B and C. A small variation in the exposure can result in a very large change in the percent of transmission.
It is desirable to have a linear relationship between the exposure level and the percent of transmission for ease of contrast control and for eventual image fidelity during development. Ideally, this relationship between the exposure level and percent transmission is linear throughout the entire exposure range. However, conventional transillumination techniques result in a relationship that is less than linear because superimposed silver grains obscure other silver grains aligned with them particularly in heavily exposed regions. In underexposed regions the solid angle subtended by the few silver grains results in undetectably low opacity.
For these reasons, todays photographic equipment is very complex because exposure must be precise. This equipment must function within the confines of the curve of the linear portion of the graph of FIG. 1 and can only extend the usable range incrementally outward along this predetermined curve.
Accordingly, it is desirable that a photo-processing method exists for extending the useful range of the curve of FIG. 1. More specifically, such a processing method would provide a more linear relationship between the percent transmission and the original exposure.
Applicant conceived and developed a technique of film image information retrieval that permits a more accurate analysis of the exposure of a film or photosensitive emulsion. Applicant's technique employs X-ray spectrometry, which is known, but which is new to photo-analysis.
The prior art uses of X-ray spectrometry is exemplified by the following U.S. patents:
U.S. Pat. No. 3,581,087 issued May 25, 1971 to Brinkerhoff.
U.S. Pat. No. 3,703,726 issued Nov. 21, 1972 to Stephenson.
The following articles are also of interest:
"X-ray Spectrometry Extends Film-Badge Dosimetry" by W. V. Baumgartner in the Aug, 1960 issue of NUCLEONICS, Vol. 18.
"Automatic Direct-Reading X-ray Spectrometry" by J. E. Moore, G. P. Happ and D. W. Stewart, Volume 33, No. 1, of the January, 1961 issue of the Analytical Chemistry.
The first article above discloses a technique for extending the over exposure range of radiation safety film badges by utilizing the ability of silver X-ray fluorescence to quantatively extend to very high radiation levels, the range from whch useful measurements can be made from radiation badge emulsions. The second article discloses a technique wherein silver X-ray fluorescences is used in quality control to measure the uniformity of silver emulsion deposited during the film manufacturing process.
These prior art methods, however, fail to recognize the applicant's problem or his solution to that problem. Neither technique recognizes the use of X-ray fluorescence as means of retrieving exposure information from silver emulsion images. Furthermore, neither technique considers the range of exposures from background fog level to maximum possible exposure level in relation to transmission radiology.